Ok! There can be many reason for this but here is one not so obvious one: you might have a mod_security2 installed on your server and hooked up to the apache installation, which causes troubles. So try disabling it by adding:



SecRuleEngine Off



to your /etc/httd/conf/httpd.conf file (restart the server after this of course).

If you are running under CPanel/WHM, you might want to change apache template /var/cpanel/templates/apache2/vhost.default - this will make sure that when a new virtual host is added to the server it will inherit the settings above and allow TinyMCE to work without having to change /etc/httd/conf/httpd.conf file again.